Such supports, e.g. as known from U.S. Pat. Nos. 3,847,438, 4,231,618 and 4,278,293, generally comprise shield plates with flat upper surfaces coming into contact with the mine roof when the plates are elevated by associated props usually designed as hydraulic jacks. As the excavation of a working face progresses, individual plates are lowered and moved forward in intermittent steps to keep pace with the advancing equipment. Each step may involve an advance by, say, a third or a fourth of the length of the plate in the direction of motion.
During such an advance, detritus from the momentarily unsupported overburden may fall onto the plate surface and remain thereon so as to prevent full-face contact between the plate and the mine roof upon the subsequent re-elevation of the support. The discontinuous contact may lead to further crumbling of the overburden and could even cause a partial collapse ahead of the shield plate.
It has already been proposed to solve this problem by providing the shield plate ahead of its leading edge with a wire mesh designed to intercept falling rock. Another suggestion entails the envelopment of the plate by a nondriven caterpillar chain. Both solutions are rather costly and neither of them has been found to be entirely satisfactory.